Social behaviour, call repertory and variation in the calls of the pool frog, Rana lessonae (Anura: Ranidae)

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Abstract
At a pond in Kottenforst, Bonn, West Germany, the calling period of the water frog Rana lessonae began on the first of May and ended by the end of June, 1983. The population is of the mixed type: 80-85 % of the individuals are classified as R. lessonae, the rest as R. esculenta. The reproductive period is subdivided into 3 successive phases: pre-spawning, spawning and post-spawning. Various exogenous factors, such as temperature, rainfall and sunlight, influence the reproductive period, and they particularly affect calling activity. The male frogs do not produce mating calls when the water temperature is below 14°C, or above 26°C. Female frogs prefer sunny places in the water during the spawning phase. Sexual activity is more intense in male R. lessonae than in male R. esculenta. Medium- and large-size male R. lessonae are more successful than small males in choosing suitable territories and in maintaining of amplexus with females. Male R. lessonae give four basic types of calls-mating call, two territorial calls and release call-and in addition two transitional calls. The basic types are given throughout the calling period, whereas the transitional calls are not. An increase in water temperature induces a decrease in call duration, intercall interval and pulse-group duration of the mating call. On the other hand, the frequencies and the repetition rate of the pulse groups are raised. Similarly, the duration of both territorial calls decreases at higher water temperatures, whereas their dominant frequencies are subjected to a notable increase. Body size has a prominent effect on the mating call. The number of pulses per pulse group and the different frequencies of the call decrease with increasing male body length. The lower pitch in the mating call produced by medium- and large-size males is assumed to play a role in their mating-success. The repetition rate of pulses and the dominant frequency in the territorial calls decrease as the body size of the frogs increases.

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